IUD Pregnancy Risk Calculator

Q: Do missing strings always mean the IUD failed?

Not always. Strings can curl into the cervix. But if you usually feel them and now can’t—or you have pain, heavy bleeding, or think the IUD passed—use backup protection and contact a clinician to check placement.

Q: Is pregnancy risk higher right after insertion?

Insertion timing and early expulsion matter. Risk is still very low overall, but it’s prudent to use backup if expulsion is suspected in the first weeks after placement.

Q: Does IUD type change risk?

Both copper and hormonal IUDs are highly effective. Differences in typical-use failure are small relative to expulsion or malposition issues—strings check and follow-up are more important drivers of risk.

Q: What if sex was within the last 5 days?

You may still be in the emergency contraception window. A copper IUD placed within 5 days is the most effective emergency contraception; pills are another option. Plan your first reliable test day if pregnancy is possible.

Using an IUD is one of the most effective ways to prevent pregnancy, but risk isn’t zero—especially early after insertion or if the device has moved. This quick calculator uses IUD type, time since insertion, whether strings are felt, possible expulsion signs, and when sex happened to estimate your per-act risk and what to do next.

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Methodology & Sources

What this estimates

This tool estimates a per-act pregnancy chance for sex while an IUD is in place and provides next-step guidance (strings check and emergency contraception window). It blends: (a) your site’s day-specific fertile-window model (used across your one-time risk tools) and (b) IUD-specific “compromise” factors (expulsion signs, strings not felt, very early post-insertion). Educational use only.

Baseline effectiveness (IUDs)

Both hormonal (levonorgestrel) and copper IUDs have typical-use failure <1% per year in large contemporary summaries and guidelines. We seed the baseline risk from your general engine using IUD method labels, which reflect these very low annual failure rates.

Sources: CDC contraceptive effectiveness materials (IUDs <1% typical-use failure); WHO/CDC guidance summarizing low failure with LARC.

Day-specific risk model (timing vs. ovulation)

Per-act risk is adjusted by cycle day weighting relative to the fertile window (peaking around the day before ovulation and day of ovulation) and very low outside those days. This mirrors the empirical day-specific conception literature and your site’s existing one-time model.

Key evidence: Day-specific probabilities of conception in prospective cohorts.

Compromised-protection flags

We apply cautious multipliers (still bounded) when protection may be compromised:

Expulsion signs: suspected/likely expulsion increases risk (temporary multipliers) and triggers the warning box.
Strings not felt: treated as possible malposition until confirmed; we advise backup and a position check.
Very early post-insertion (first month): small additional risk due to higher expulsion risk shortly after placement.

Sources for clinical handling: CDC U.S. Selected Practice Recommendations (SPR) and ACOG LARC guidance (evaluation of missing strings/expulsion).

Emergency contraception window

The tool shows whether you are within an EC window based on the intercourse date:

Levonorgestrel (LNG) pills: most effective within 3 days (72 h) after sex.
Ulipristal acetate (UPA): up to 5 days (120 h) after sex.
Copper IUD as EC: the most effective EC when appropriate, typically up to 5 days after sex.

Sources: ACOG EC guidance; CDC SPR EC section.

Computation notes

Engine: Calls your site’s general per-act engine with the selected IUD method, then re-weights by day-of-cycle.
Bounds: Results are clamped to avoid unrealistic extremes and shown as a percentage with a qualitative band.
Flags: “Strings not felt” or expulsion signs raise the displayed risk band and show safety guidance.

Limitations

Population averages cannot reflect individual device position, insertion technique, postpartum status, medications, or clinic-specific practices. If you suspect expulsion or pregnancy symptoms, seek clinical assessment.

How to read your result

Lower risk: Strings felt, no expulsion signs, and several months since insertion.
Higher risk: Can’t feel strings, device or clots passed, severe cramps/bleeding, or very recent insertion.
Timing matters: If sex was within the fertile window and there are expulsion signs, treat risk as higher.

What to do now

Strings check: If you normally feel them and now can’t—or you suspect expulsion—use backup protection and contact a clinician.
Emergency window: If sex was within the last 120 hours (5 days), emergency contraception may help. A copper IUD placed within 5 days is the most effective EC; pills are another option.
Testing: If risk wasn’t negligible, plan a urine test on the first reliable day and repeat if negative and period is still late.

Heads-up: This tool provides estimates for a single encounter. It doesn’t diagnose expulsion or confirm correct placement—only an exam or imaging can do that.

How to read your result

Lower risk: Strings felt, no expulsion signs, and several months since insertion.
Higher risk: Can’t feel strings, device or clots passed, severe cramps/bleeding, or very recent insertion.
Timing matters: If sex was within the fertile window and there are expulsion signs, treat risk as higher.

What to do now

Strings check: If you normally feel them and now can’t—or you suspect expulsion—use backup protection and contact a clinician.
Emergency window: If sex was within the last 120 hours (5 days), emergency contraception may help. A copper IUD placed within 5 days is the most effective EC; pills are another option.
Testing: If risk wasn’t negligible, plan a urine test on the first reliable day and repeat if negative and period is still late.

Heads-up: This tool provides estimates for a single encounter. It doesn’t diagnose expulsion or confirm correct placement—only an exam or imaging can do that.

FAQ

Do missing strings always mean the IUD failed?

Not always. Strings can curl into the cervix. But if you usually feel them and now can’t—or you have pain, heavy bleeding, or think the IUD passed—use backup protection and contact a clinician to check placement.

Is pregnancy risk higher right after insertion?

Insertion timing and early expulsion matter. Risk is still very low overall, but it’s prudent to use backup if expulsion is suspected in the first weeks after placement.

Does IUD type change risk?

Both copper and hormonal IUDs are highly effective. Differences in typical-use failure are small relative to expulsion or malposition issues—strings check and follow-up are more important drivers of risk.

What if sex was within the last 5 days?

You may still be in the emergency contraception window. A copper IUD placed within 5 days is the most effective EC; EC pills are another option. Plan your first reliable test day if pregnancy is possible.

Sources

CDC. U.S. Selected Practice Recommendations for Contraceptive Use, 2024 — management of IUD issues, counseling, and follow-up.
ACOG. Long-Acting Reversible Contraception (IUD and Implant) — patient guidance on effectiveness and strings checks.
WHO/Family Planning Handbook — IUD chapter: IUD — expulsion signs, checking strings, and when to seek care.
ACOG. Emergency Contraception — copper IUD within 5 days as most effective EC.

Educational info only; not medical advice. For suspected expulsion or pregnancy symptoms, contact a clinician promptly.